clicking sound

Hello, I have a 1999 Chevy Lumina (car) I have had it almost 2yrs, the last 2yrs have been GREAT with this car, no problems at all. Untill recently, I got in the car to come home from work, put keys in the ignition, and instead of turning on, it made a clicking sound. Although my lights were still on. Could anybody please tell me what this could be? Also, a "Security" light has been on, what does that mean? Thanks for your help look forward to hearing some answers soon!

I would suspect a bad battery or dirty/corroded battery cables. Check for any green corrosion around the battery cables where they attach to the battery, as well as at the jump start post on the drivers side strut tower. If no corrosion is present, you probably need a battery.